1. Field of the Invention
This invention generally relates to fiber optic networks, and more specifically, the invention relates to fiber optic networks that carry multiple optical signals at multiple wavelengths. Even more specifically, the invention relates to methods for, and to a monitor for, compensating for changes in the signals of such fiber optic systems.
2. Prior Art
The Internet-driven bandwidth demand explosion has accelerated the introduction of wavelength division multiplexing and photonic components for optical networks, evolving from long-haul telecom “core” and “edge” to the shorter distance “Acess” and metropolitan segments with differing requirements for lower cost and multiple protocols and or businesses sharing the optical spectrum. The long-haul application requires multiple optical amplifiers, while the short-haul requires bandwidth provisioning management.
Fiber optic dense wavelength division multiplexing (DWDM) systems have also found increasing applications in metropolitan area datacom networks (MANs). The DWDM equipment is often combined with optical repeaters, amplifiers, switches, and other networking elements. This creates a fundamental problem, in that wavelength channels cannot be added or dropped from the fiber link without impacting all of the remaining channels in the network. For example, in a 32 wavelength DWDM system, the optical power launched by each wavelength tuned laser is not equal, and furthermore each wavelength experiences different optical attenuation as it passes through the network elements (these include wavelength add/drop filters, optical amplifiers, interconnect switches, etc.). In other words, the optical transfer function (OTF) of a DWDM network is not constant over wavelength; it varies significantly depending on the number of wavelengths in use at a given time. Adding or dropping one or more wavelengths requires that the rest of the network be adjusted to compensate for the change in optical power; failure to do this may give rise to nonlinear optical effects or impact the link budget and target bit error rate of the system.
All optical networks encode the information to be transmitted and received. A stable optical power level (“D.C.”) is vital to set and maintain the code's threshold for digital data: above the threshold is a “one” and below the threshold is a “zero.” Bouncing this code-threshold value results in bit errors, corrupted data, and, frequently, uncontrolled quality of service (QoS) as header-addresses for information packets are lost. A stable optical power threshold for lightwave communication is, therefore, essential to optical network operation.
To address this problem, various schemes have been proposed to design optical control modules (OCMs) which compensate for the nonuniform OTF of the network. As a simple example, part of the optical power at each wavelength may be sampled using an optical splitter and detector, with the resulting control signal fed back to adjust equalizers in the optical amps or laser transmitter power in the DWDM equipment. The accuracy of these systems is often poor, and the implementation cost can be high for many wavelengths with dense spacing.